Abstract: The Antarctic ichthyofauna evolved in isolation under the selective pressure of low and stable temperature during the last few million years. The elevated levels of fluoride present in the carapace of Antarctic krill do not appear to have any toxic effects on the organisms that feed on it. However, as far as we are aware there are no studies about the effect of this halogen on Antarctic fish metabolism particularly when subjected to thermal and hypo-saline stress. The increased human activity in this region has resulted in the Antarctic protection protocol that calls for increased research for monitoring the environment. The discharge of sewage in the region has raised questions about its possible effects on the marine ecosystems. The present study investigates the effects of a) fluoride on the hepatic metabolism on N. rossii; b) sewage from the Antarctic station Commandant Ferraz (EACF) on the hepatic, renal, gill and muscle metabolism as well as on the plasma constituents in the fishes N. rossii and N. coriiceps. The activity of the enzymes involved in the energy metabolism, antioxidant defense and the catabolism of L-arginine were used as metabolic indicators. The hepatic capacity to metabolize xenobiotics was evaluated by the enzyme ethoxyresorufin-O-deethylase (EROD). Experiments involving tropic fluoride were conducted using a combination of two temperatures (0 and 4oC), two salinities (35 and 20 psu) and two trophic fluoride (with and without) conditions. Experiments involving sewage were conducted for a) short term (96 hours) with both the species with sewage diluted to 0.5% (v/v); b) long term (25 days) with only N. rossii, but with the sewage diluted to 0.05% (v/v) and 0.5% (v/v). The hepatic metabolism of N.rossii, was not modulated by trophic fluoride in any of the experimental conditions. This is in contrast to non-Antarctic fishes were elevated trophic fluoride cause histo-pathologic lesions and altered the hepatic metabolism. In experiments with sewage, with both species a) the enzymes involved in the energy metabolism of both gills and kidneys were positively modulated, though for a short term there was a reduction in hexokinase; b) the liver elevated the glyconeogenic and glycogenolitic potential and retained the glycemic levels in N. rossii, but not in N. coriiceps which developed hypoglycemia; c) the gill osmoregulation energy potential marker was not altered in either species; d) N. rossii had higher plasma levels of energy substrates and non-protein electrolytes compared to N. coriiceps; e) muscle tissue reduced the glucose potential activator, but elevated the ATP aerobic generation potential in both species; f) the elevation of anti oxidant defense enzymes was most evident in the kidneys and liver; g) the gills and kidneys positively modulated the arginine metabolism for the short term; h) Hepatic EROD were not modulated in N. coriiceps, but was reduced in N. rossii for both the long and short term. In both species, diluted sewage altered the energy metabolism enzymes, particularly in the gills and kidneys, indicating a large increase in the energy demands in tissues of these two organs. Even though the two species N. rossii and N. coriiceps, are philogenetically close, they have distinct response on exposure to sewage from EACF. The study shows that the enzymes involved in the energy metabolism of these two fish species can be candidate biomarkers of sewage pollution in the Antarctic.